In recent years, an energy saving trend on industrial products has been strong in view of environmental problem, rise of crude oil price, etc. The field of construction vehicles, work vehicles, etc. in which hydraulic drive systems using Diesel engines have heretofore played a central role is also on the same trend so that there has been an increasing number of instances using electrification to increase efficiency and improve energy saving.
When, for example, a drive portion of a vehicle is electrified, that is, a drive source thereof is set as an electric motor, lots of energy saving effects such as engine's high efficiency drive (hybrid model), improvement in power transmission efficiency, recovery of regenerative electric power, etc. in addition to reduction of exhaust gas can be expected. In the aforementioned field of construction vehicles, work vehicles, etc., electrification of forklifts has been most advanced, and “electric-powered forklifts” in which motors are driven using electric power of batteries have taken the lead ahead of other vehicles and have been put into practical use. Consecutively to this, “hybrid vehicles” using combination of Diesel engines and electric motors in hydraulic excavators, engine type forklifts etc. have recently started being produced.
Among these construction machines and work vehicles on the trend toward ecology and energy saving using electrification as described above, wheel loaders are vehicles which can be expected to have a comparatively large effect on reduction of fuel consumption when the vehicles are made hybrid. As shown in FIG. 8, a background-art wheel loader has a travel portion (wheel portion) and a front hydraulic work portion (lift/bucket portion) as main movable portions so that while motive power of an engine 1 is transmitted to tires 13 through a torque converter 2 and a transmission (T/M) 3 to make the vehicle travel, the hydraulic work device 5 in the vehicle front portion is driven by a hydraulic pump 4 so as to excavate and carry soil etc.
Power transmission efficiency of the torque converter used in the background art is inferior to power transmission efficiency based on electricity. When a travel drive portion of the aforementioned background-art wheel loader is electrified, power transmission efficiency from the engine can be therefore more improved. Further, since the wheel loader repeats travel start and stop operations frequently during work, it is expected to recover regenerative electric power at the time of braking from a drive motor when the travel portion is electrified. It is said that fuel consumption can be generally reduced by about a few tens of percent when part of the drive device of the current wheel loader is electrified and hybridized in this manner.
A method for controlling a hybrid system of such a work vehicle is, for example, shown in Patent Literature 1. This Patent Literature 1 has given disclosure about a system in which a control mode is selected in accordance with the noise level of a hybrid work vehicle. Specifically, as for a drive system in a low idle state of an engine, first, the engine is fixed to the low idle state while the shortage of a pump output obtained only by driving the engine is supplemented by an electric motor if necessary, so that a pump output set by a hand throttle can be obtained. Next, it is checked whether the electric motor is being driven or not, and a work machine is driven in accordance with an operation signal from a work machine lever when the electric motor is being driven.
On this occasion, when the electric motor is under suspension, the electric motor is driven at a fixed rotation speed so that the work machine can be driven by the engine and the electric motor. After that, the amount of charge in a battery is calculated, and it is determined whether the battery is empty or not based on the amount of charge. Then, when the battery is empty, a warning is outputted while the electric motor is suspended and the hydraulic pump is driven only by the low idle of the engine. The electric motor is suspended when no operation signal from the work machine lever is inputted, and the engine is suspended when the battery is full of charge. On the other hand, when the battery is not full of charge, the battery is charged by a generator.
In this manner, according to the technique described in Patent Literature 1, it is possible to achieve a low-noise drive system in which the engine is set in a low idle state or an idling stop state in accordance with the noise level of the vehicle.